1. Field of the Invention
The present invention relates to an illumination apparatus useful for inspecting a surface of an object which has a three-dimensional shape, for example, a cylindrical shape to detect presence of a defect such as a surface flaw or a dent/projection or non-uniformity of the surface, and an appearance inspection apparatus which includes such an illumination apparatus.
2. Description of the Related Art
Various appearance inspection apparatuses (object surface inspection apparatuses) that are used for detecting presence of a surface defect, such as a flaw, a dent/projection, and adhesion of a foreign substance, of an object to be inspected has been proposed.
As an appearance inspection apparatus, for example, an inspection apparatus is known that irradiates a surface of an object to be inspected with uniform projection light and detects its surface defect based on obtained distribution of light (or its image), which is composed of specular reflection light and diffused reflection light from the surface of the object.
Japanese Patent Application Laid-Open No. 2007-334216 (US 2007/0292088) discusses an appearance inspection apparatus that efficiently evaluates an uneven surface state of an object to be inspected caused by distribution of surface dents and projections.
It is conventionally known that by using tangential illumination light which is provided close to the surface and incident on the surface at an angle of approximately 90 degrees, the surface state of the object can be accurately detected without being affected by microstructure of the object at the surface. This method is especially effective in detecting a foreign substance or a dust attached to the surface.
Japanese Patent Application Laid-Open No. 2004-194689 and Japanese Patent Application No. 2000-315256 discuss an apparatus that inspects appearance of an object which has a curved inspection surface rather than a flat surface.
When carrying out appearance inspection of a surface of an object, it is important to efficiently evaluate the uneven surface state caused by the distribution of surface dents and projections, a dust or a foreign substance attached to the surface, and surface roughness (hereinafter collectively called as “surface defect”).
Generally, when a surface defect is inspected using an image of an inspection surface, the surface defect can be accurately and easily detected (observed) under an optimum lighting condition that is changed according to a type of the surface defect, for example, if the inspection surface is uniformly illuminated with light and an incident angle of the light with respect to the inspection surface is large.
In order to accurately inspect a surface defect of an object which has a three-dimensional shape such as a cylindrical object or a spherical object, it is important to efficiently illuminate the inspection surface with light of even light distribution and of a large incident angle.
However, it has been difficult for the conventional illumination apparatus to illuminate a surface of a three-dimensional shape object under a lighting condition that is adjusted according to a type of the surface defect.
More particularly, it has been very difficult to efficiently inspect different types of surface defects without being affected by a surface structure of the inspected object having a three-dimensional shape.